Straight-banded chalcedony. Almost all solid black 'onyx' in jewellery is dyed grey chalcedony.
Brazil, Uruguay, India, Mexico, Germany (Idar-Oberstein), Scotland3 references
Onyx proper is chalcedony with straight parallel bands, as distinct from agate's concentric curved ones. Sardonyx is the brown-and-white version. Both are natural and both are relatively uncommon in good quality.
What the jewellery trade means by onyx is different: solid, uniform, glossy black chalcedony, and essentially all of it is dyed. Grey chalcedony is soaked in a sugar solution and then treated with sulphuric acid, which carbonises the sugar in the pores and turns the stone permanently and evenly black. The treatment is stable, ancient — Roman jewellers did it — and effectively never disclosed, because in this corner of the market 'onyx' has come to mean the treated material.
It is worth knowing mainly so that a completely even black stone is not mistaken for something rarer. Test hardness: dyed chalcedony is 7 and scratches glass, where black glass, jet and black obsidian are all softer.
Black onyx · Sardonyx · Banded onyx
Establish which of these you are holding before judging it. A genuine piece read against another reference's rules will look wrong when it is not.
These apply to this model and not to the rest of Agate. Work them alongside the maker's general checks, not instead of them.
Uniform, glossy, unbanded black is the product of a sugar-and-acid treatment that has been standard for two thousand years. Natural onyx is banded; solid black natural chalcedony of that evenness is rare.
Dyed chalcedony is hardness 7 and marks glass. Obsidian at 5–5.5 and jet at 2.5–4 do not. Jet is also far lighter and streaks brown.
Cheap. Dyed black onyx is a few dollars a cabochon.
An orientation for honest, unrestored condition — never an appraisal, and never a quotation.
Lovenhall StoneThe tests tell you what it is. The instrument reads a photograph and offers a name to start from — then hands you back to the tile and the coin, which are what settle it.